L. N. KOMISSAROVA, B. I. POKROVSKII

ON THE THERMAL STABILITY OF ScF₃ AND ITS INTERACTION WITH MgF₂

(Presented by Academician V. I. Spitsyn, 22 XI 1962)

Scandium fluoride may be used to obtain free scandium and its magnesium alloys by reduction with metallic magnesium. However, the literature data on the physicochemical properties of ScF₃ are very limited. It is known only that this compound is a stable crystalline substance with a melting point \(>1000^\circ\) (¹). It crystallizes in a rhombohedral (pseudocubic) lattice with parameters \(a = 4.022 \pm 0.004\) Å and \(d(\mathrm{Sc—F}) = 2.01\) Å (²).

Fig. 1. Dependence of the change in weight of scandium fluoride: a—on temperature, b—on time at temperatures 700° (1), 800° (2), 900° (3)

Fig. 2. Debye patterns (Co anode): a—Sc₂O₃; b—partially hydrolyzed ScF₃; c—ScF₃; d—alloy with 64.01% ScF₃; e—alloy with 10.6% ScF₃; f—MgF₂

We have determined the melting point of ScF₃ and have studied its thermal stability and its interaction with magnesium fluoride in the molten state. The work used scandium fluoride obtained from spectrally pure scandium oxide by treatment with a 40% solution of hydrofluoric acid of “chemically pure for analysis” grade. The precipitate of scandium fluoride was washed with water and absolute alcohol and dried for 4 hours at a temperature of 450°. Chemical analysis of the preparation showed that its composition corresponded exactly to the formula ScF₃. Magnesium fluoride was obtained by reacting magnesium carbonate of “chemically pure” grade likewise with a 40% solution of hydrofluoric acid. The investigation of the thermal stability of scandium fluoride was carried out using continuous-weighing balances in the interval 20–1000°. The heating rate averaged 5 deg/min. The weighed sample of the preparation under study was ~0.2 g.

Figure 1a gives a typical curve of the change in weight of ScF₃ with temperature. The slight decrease in the weight of the preparation upon heating in air-

...in air is already observed at about 400°; above 650° a sharp change in weight occurs, ending at 950°. Samples of scandium fluoride whose weight had decreased, respectively, by 5, 10, 20, 25, and 32.3% were subjected to X-ray phase analysis. In the debyegrams obtained, lines were found corresponding only to scandium fluoride and oxide. The formation of oxyfluorides, as might have been expected by analogy with the rare-earth elements ($^3$), is not observed in this case: as a result of hydrolysis, scandium fluoride is converted directly into the oxide. The intermediate products are mixtures of scandium oxide and fluoride (Fig. 2). The rate of the process of transformation of $\mathrm{ScF_3}$ into $\mathrm{Sc_2O_3}$ depends to a considerable extent on temperature. In Fig. 1b the isotherms of hydrolysis of scandium fluoride are given, from which it is seen that at 900°, for complete completion of the process, 2.5 h is sufficient; at 800°, 5.3 h; and at 700°, as much as 11 h is required.

Fig. 3. Heating and cooling curves: a — $\mathrm{ScF_3}$; b — alloy with 64.01% $\mathrm{ScF_3}$; c — alloy with 9.57% $\mathrm{ScF_3}$; d — $\mathrm{MgF_2}$

The melting temperature was determined by the method of thermal analysis from heating curves (Fig. 3a), recorded on a Kurnakov pyrometer. Carrying out the experiment was complicated by the slight thermal stability of $\mathrm{ScF_3}$ in air at temperatures above 400°. Therefore the substance was placed in a platinum crucible, tightly closed with a lid that had an opening for the thermocouple. Temperature was measured with a platinum–platinum-rhodium thermocouple. The samples were melted in a vertical silite furnace at a heating rate of 105°/min, i.e., a temperature of 1600° was reached within 15 min.

As a result of the investigation carried out, it was established that scandium fluoride melts at a temperature of $1530 \pm 20^\circ$ and undergoes a polymorphic transformation at $1350 \pm 20^\circ$. Interactions in the $\mathrm{ScF_3}$—$\mathrm{MgF_2}$ system were studied by methods of thermal and X-ray phase analysis. Thermal analysis was carried out from cooling curves recorded on a Kurnakov pyrometer (Fig. 3). The mixtures of fluorides were placed in tightly closed platinum crucibles; heating was carried out in a silite furnace. The results of the investigation are presented in Table 1 and in Fig. 4.

Fig. 4. Phase diagram of the $\mathrm{ScF_3}$—$\mathrm{MgF_2}$ system

In the $\mathrm{ScF_3}$—$\mathrm{MgF_2}$ system, on the magnesium fluoride side there is a narrow region of solid solutions (not more than 5 mol.% $\mathrm{ScF_3}$). The presence of the latter is confirmed by a lowering of the temperature of the polymorphic transformation of magnesium fluoride from 960° in $\mathrm{MgF_2}$ to 840° in alloys containing more than 5 mol.% $\mathrm{ScF_3}$. In addition, a decrease is observed in the lattice parameter of the solid phase based on $\mathrm{MgF_2}$ with increasing concentration of $\mathrm{ScF_3}$. For magnesium fluoride, the para-

parameters \(a\) and \(c\), respectively, are \(4.613 \pm 0.005\) kX and \(3.081 \pm 0.005\) kX; an alloy with 3.6 mol.% \(\mathrm{ScF_3}\) has \(a = 4.602 \pm 0.005\) kX and \(c = 3.057 \pm 0.005\) kX. With a further increase in the concentration of scandium fluoride, saturation of the solid solution occurs and the lattice parameters remain constant; the parameters of the solid solution containing 10.6 mol.% \(\mathrm{ScF_3}\) are: \(a = 4.595 \pm 0.005\) kX and \(c = 3.041 \pm 0.005\) kX.

Scandium fluoride and the solid solution based on \(\mathrm{MgF_2}\) form a eutectic with one another. The composition of the eutectic point corresponds to 34 mol.% \(\mathrm{ScF_3}\), and the melting temperature of the eutectic mixture is 1095°.

No formation of intermediate phases is observed in the system, which is confirmed by the X-ray phase-analysis data. In Fig. 2 \((b, c, d, e)\), for comparison, Debye diagrams are presented for magnesium fluoride, scandium fluoride, and fluoride alloys with 10.6 and 64.01 mol.% \(\mathrm{ScF_3}\). Both alloys consist of two components: scandium fluoride and a solid solution based on magnesium fluoride (the diffraction lines in the Debye diagrams shown for \(\mathrm{MgF_2}\)—solid solution and pure \(\mathrm{MgF_2}\) practically coincide because of the small difference in the angle \(\theta\) for the corresponding lines).

Table 1

Results of the investigation of the phase diagram of the \(\mathrm{ScF_3}\)—\(\mathrm{MgF_2}\) system

The polymorphic transformation of \(\mathrm{MgF_2}\), recorded by us at 960°, is traced in the alloys of the system up to 49 mol.% \(\mathrm{ScF_3}\), and its temperature decreases to 840°.

The polymorphism of scandium fluoride at 1350° appears throughout the entire concentration range and is marked by a slight bend in the liquidus curve.

Thus, the \(\mathrm{ScF_3}\)—\(\mathrm{MgF_2}\) system belongs to type IV of phase diagrams according to Roozeboom.

Moscow State University
named after M. V. Lomonosov

Received
14 XI 1962

CITED LITERATURE

1. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, 5, 1946, p. 480.
2. W. Z. Nowacki, Kristallogr., 101, 273 (1939).
3. A. I. Popov, A. E. Kondson, J. Am. Chem. Soc., 76 (15), 3921 (1954).